1. Field of the Invention
This invention relates to a heat-sensitive stencil master making apparatus in which a stencil master is made by imagewise perforating a heat-sensitive stencil master material by a thermal head.
2. Description of the Related Art
There has been known a heat-sensitive stencil master making apparatus in which a thermal head having an array of heater elements is pressed against the thermoplastic film side of a heat-sensitive stencil master material while selectively energizing the heater elements, thereby perforating the thermoplastic film in a pattern representing image data.
FIG. 24 shows an example of such a stencil master making apparatus. In FIG. 24, the stencil master making apparatus 90 of this example comprises a thermal head 4 having an array of a plurality of heater elements 5 (only one is visible in FIG. 24), and a platen roller 3. A heat-sensitive stencil master material 1 is conveyed in the direction of arrow A when the platen roller 3 is driven by an electric motor (not shown) and passed between the platen roller 3 and the thermal head 4 with the side of a thermoplastic film 1a of the stencil master material 1 facing the thermal head 4. Thus the heater elements 5 of the thermal head 4 are pressed against the thermoplastic film 1a of the stencil master material 1 and the thermoplastic film 1a is perforated in a pattern representing image data by selectively energizing the heater elements 5 by a head drive means (not shown).
Each of the perforations is formed in the following steps. When a heater element 5 starts to be energized and heated, the temperature of the part of the thermoplastic film 1a in contact with the heater element 5 is elevated. Since the temperature of the heater element 5 is the highest at the center thereof, the temperature of the thermoplastic film 1a is maximized at the part in contact with the center of the heater element 5. When the temperature of this part reaches a perforation generation temperature to be described later, a small perforation is generated at this part. The small perforation is enlarged over an area circumscribed by an isothermal line at a shrinkage initiation temperature to be described later. After the heater element 5 is de-energized, the area circumscribed by the shrinkage initiation temperature line once enlarges and then narrows, and accordingly enlargement of the perforation stops.
When perforations are to be formed, each heater element 5 is generally applied with target power (more specifically, a voltage calculated on the basis of the mean resistance for all the heater elements 5 of the thermal head 4 and target power to be applied to the heater element 5) continuously for a predetermined time as shown in FIG. 25A. The power applied to each heater element 5 will be referred to as xe2x80x9cthe heater drive powerxe2x80x9d and the time for which the heater drive power is applied to the heater element 5 will be referred to as xe2x80x9cthe duration of heater drive powerxe2x80x9d, hereinbelow.
When the heater drive power is applied to the heater element 5, the surface of the heater element 5 has a temperature distribution such that the temperature is the highest at the center of the heater element 5 and lowers as the distance from the center increases as shown in FIG. 26. The temperature distribution changes depending on the shape and structure of the heater element 5 and the heater drive power and/or the time, and is an important factor which affects the shape of the perforation. In the following description, the temperature at the center of the surface of the heater element 5 is taken as a representative of the temperature of the heater element 5, and xe2x80x9cthe temperature of the heater element 5xe2x80x9d as used hereinbelow means the temperature at the center of the surface of the heater element 5 unless otherwise noted.
So long as the shape and/or the structure are the same, the surface temperature distribution is similar, and accordingly, the temperature of the heater element 5 represents the state of heating of the heater element to some extent.
Since the heater drive power is of a square wave as shown in FIG. 25A, the temperature of the heater element 5 changes like an exponential function and asymptotically approaches a certain temperature with time as shown in FIG. 25B while the heater element 5 is energized. That is, the temperature of the heater element 5 is low at the beginning of application of the heater drive power, is monotonically increased and is maximized at the end of the application.
In order to improve quality of printed images, it is required that the perforations are as uniform as possible in shape. Nonuniformity in shape of the perforations is caused partly for systematic reasons and partly for random reasons. For example, the systematic reasons include the ambient temperature (when the ambient temperature is high, the perforations are enlarged, and vice versa), heat accumulation (the perforations are small at the beginning of stencil master making, and are gradually enlarged as the stencil master making process progresses due to accumulation of heat), common drop (when perforations are formed over a wide area, perforations are apt to become smaller in the end portions than in the middle portion in the main scanning direction, where the line resistance is higher), and the like. The random reasons include fluctuation in the temperature of the heater elements, dispersion of fibers in the support sheet of the stencil master material, nonuniformity in the state of contact between the thermoplastic film of the stencil master material and the heater elements due to surface roughness of the thermoplastic film, and the like. Unlike in other thermal recording such as those using heat-sensitive paper or thermal transfer, fluctuation in perforation size in the heat-sensitive stencil master is greatly affected by the random reasons. Accordingly, it is especially required in heat-sensitive stencil master making that nonuniformity in the perforation size is suppressed. Further improvement in printing durability and accuracy in the printing position, shortening the stencil master making time and the like are required. In order to meet these requirements, there have been made various studies on the material of the thermoplastic film.
For example, as disclosed in Japanese Patent Publication No. 2507612, there has been proposed use of thermoplastic film having two melting peaks in order to stabilize the shape of perforations. That is, by use of such thermoplastic film, generation of perforations is quickened by virtue of the resin component having the lower melting peak and the shape of the perforations is stabilized by virtue of the resin component having the higher melting peak. When a perforation is formed in heat shrinkable film, the perforation is generally enlarged over an area circumscribed by a shrinkage initiation temperature line (an isothermal line at a shrinkage initiation temperature) and fluctuation in size of the perforations is smaller as the temperature gradient near the shrinkage initiation temperature line is larger and as the degree to which the heat-shrinkage factor rises beyond the shrinkage initiation temperature increases. Actually, the heat-shrinkage temperature range of resin whose melting peak temperature is high is in a relatively high temperature range, and accordingly the temperature gradient near the shrinkage initiation temperature line is large. This stabilizes the shape of the perforations. However when the thermoplastic film is formed only of such high melting peak resin, sensitivity to perforation of the thermoplastic film, that is, the performance in forming perforations of a required size with low power, deteriorates. Accordingly in order to keep sufficient the sensitivity to perforation of the thermoplastic film, resin whose melting peak temperature is low is added. However, when the proportion of such low melting peak resin is increased giving precedence to the sensitivity to perforation, stability of the shape of the perforations deteriorates and when the proportion of the high melting peak resin is increased giving precedence to the stability of the shape of the perforations, the sensitivity to perforation deteriorates.
Further, as disclosed in Japanese Unexamined Patent publication No. 62(1987)-282984, there has been proposed thermoplastic film whose heat of crystalline fusion, energy of fusion and melting point are defined. Generally the sensitivity to perforation of heat-shrinkable resin is higher as the heat of crystalline fusion is smaller, the energy of fusion is smaller and the melting point is lower. However, the above identified Japanese Unexamined Patent publication says that the printing durability, that is, the number of copies which can be printed before the thermoplastic film is broken, is larger as the heat of crystalline fusion is larger, the energy of fusion is larger and the melting point is higher unless the thermoplastic film is extremely large in thickness. That is, the printing durability and accuracy in the printing position which depends upon the printing durability conflict with the sensitivity to perforation.
As can be understood from description above, thermoplastic film which contributes to stabilization of the shape of perforations and/or is excellent in mechanical strength, thereby contributing to improvement in the printing durability and the accuracy in the printing position, is generally lower in the sensitivity to perforation. Further even thermoplastic film which is high in sensitivity to perforation deteriorates in its sensitivity to perforation when the thickness thereof is increased with the aim of improving the printing durability or the accuracy in the printing position. In the case of thermoplastic film which is low in sensitivity to perforation, the energy supplied to the heater elements must be larger in order to obtain a desired shape of the perforations. The energy supplied to the heater element is a value obtained by time-integration of the heater drive power.
The energy supplied to the heater elements can be increased by increasing the heater drive power, increasing the duration of heater drive power, or increasing both the heater drive power and the duration thereof. FIG. 27 shows change with time of the temperature of the heater element when the energy supplied to the heater element is increased by a given amount by the three methods from energy represented by line a which is proper to conventional high-sensitive thermoplastic film. That is, line a represents the change of the temperature of the heater element when the heater drive power and the duration thereof are set to be suitable for thermoplastic film which is high in sensitivity to perforation. Line b represents the change of the temperature of the heater element when the heater drive power is increased as compared with that for line a with the duration of the heater drive power kept unchanged. Line c represents the change of the temperature of the heater element when the duration of heater drive power is increased as compared with that for line a with the heater drive power kept unchanged. Line d represents the change of the temperature of the heater element when both the heater drive power and the duration of heater drive power are increased as compared with those for line a.
It has been empirically known that so long as the total energy is unchanged, that is, the product (heater drive power x the duration of heater drive power) is the same, substantially the same shape of perforations can be obtained when the heater drive power or the duration of heater drive power is in xc2x1 30% of that of a reference combination of the heater drive power and the duration of heater drive power. Further it has been empirically known that so long as the amounts of energy applied to the heater element are equal to each other, the peaks of lines b to d (the maximum temperatures of the heater element) are substantially on a curve which descends rightward as shown by the broken line in FIG. 27. Since the peak of line c is higher than that of line a, the peaks of lines b and d are also higher than that of line a.
In order to shorten the stencil master making time, it is generally necessary to shorten the duration of heater drive power. For this purpose, it is necessary to increase the heater drive power applied to the heater elements. FIG. 28 shows change with time of the temperature of the heater element when the heater drive power applied to the heater element is changed. That is, line a represents the change of the temperature of the heater element when the heater drive power and the duration thereof are set to be suitable for thermoplastic film which is high in sensitivity to perforation. Line b represents the change of the temperature of the heater element when the heater drive power is increased and the duration of heater drive power is shortened as compared with those for line a. Also in this case, the peaks of lines a and b are on a curve which descends rightward as shown by the broken line in FIG. 28, and accordingly the peak of line b is higher than that of line a.
Further, as disclosed, for instance, in Japanese Unexamined Patent publication Nos. 62(1987)-51465, 62(1987)-227663 and 4(1992)-85050, it has been proposed in the field of thermal transfer recording to apply to the heater elements chopped pulses in place of square pulses in order to improve printing quality. This is for preventing sticking of ink sheets, keeping a temperature suitable for printing and controlling the printing density.
As can be understood from description above, it is necessary to set a condition of application of heater drive power which can heat the heater elements to a temperature higher than the conventional temperature in order to satisfy the requirements from the quality of printed images.
However if the heater elements undergo an excessively high temperature, oxidation of the heater elements is promoted and the heater elements deteriorate in their heat generating performance (generally referred to as xe2x80x9cdeterioration of the heater elementsxe2x80x9d) or are broken. As some of deterioration modes of heater elements are described, for instance, in xe2x80x9cThermal Head Arrayxe2x80x9d by Uyama (an extra number of Shashinkougyou, edited by Academy of Electrophotography, Imaging, Part 3, pp.45 to 54, Shashinkougyou Shuppan, 1988), deterioration modes of the heater elements due to the temperature of the heater elements themselves include a glaze layer breaking mode, an oxidation mode and a crack mode. The glaze layer breaking mode occurs when the temperature of the heater elements exceeds 600 to 700xc2x0 C., the softening point of the glaze layer, and will not occur under practical perforating conditions. The oxidation mode occurs when the heater elements are continuously operated at a temperature slightly higher than their service temperature. When the heater elements are continuously operated at a temperature slightly higher than their service temperature, they are oxidized and their resistance increases and finally the heater elements becomes incapable of generating heat. According to our experiment, in the case where a thin film thermal head is operated at a cycle of 2.5 msec, tendency of deterioration of heater elements begins to appear when the temperature of the heater elements slightly exceeds 400xc2x0 C. The crack mode occurs when the heater element undergoes abrupt changes of temperature. That is, when the heater element is subjected to abrupt changes of temperature, the protective layer is cracked due to thermal shocks or displacement of layers due to difference in thermal expansion coefficient and when the protective layer is once cracked, oxidation of the heater element is rapidly promoted.
In a practical heat-sensitive stencil master making process, the thermal head is operated under the conditions that the peak temperature of the heater elements is 300 to 400xc2x0 C. and the cycle time is 2 to 4 msec. Unlike in recording on heat-sensitive paper, these conditions are very severe from the viewpoint of durability of the heater elements in heat-sensitive stencil master making process.
In the case of heat-sensitive paper F50SS available from FUJI PHOTO FILM Co., the data on which is described as a representative of properties of sensitivity of heat sensitive paper in xe2x80x9cDirect Thermal Recording Paperxe2x80x9d by Usami and Igarashi (an extra number of Shashinkougyou, edited by Academy of Electrophotography, Imaging, Part 3, pp.165 to 176, Shashinkougyou Shuppan, 1988), color begins to be developed at about 80xc2x0 C. and the density of the color is saturated at about 110xc2x0 C.
In the case of fusion type thermal transfer recording, the melting point of typical fusion type thermal transfer ink is 65 to 75xc2x0 C. as disclosed in xe2x80x9cTransfer Type Color Thermal Recording Mediaxe2x80x9d by Seto, Shimazaki and Kondou [Papers for 1st Non-Impact Printing Technique Symposium, 3 to 8, P61 (1984)].
Further, in the case of sublimation type thermal transfer recording, the sublimation temperature of sublimation dye is generally in the range of 140 to 200xc2x0 C. though varies depending on the color of the dye as shown in xe2x80x9cTechnique of Video-Printerxe2x80x9d by Hori [Magazine of Academy of Electrophotography, 29-p1, P77, (1990)]. As for the temperature of the heater elements of the thermal head in sublimation type thermal transfer recording, the change with time of the temperature of the center of the surface of heater element is shown in xe2x80x9cPrinting Properties of Sublimation Type Thermal Printerxe2x80x9d by Mochizuki and Saitou [Briefs of Lectures on Thermal Technology in Japanese Academy of Mechanics, vol. 1989, P120, (1989)] and the peak of the temperature is shown to be about 280xc2x0 C. The ratio of sublimation of the dye under the condition shown is about 70%, which is more than sufficient for transfer.
To the contrast, in the case of heat-sensitive stencil master making, it is necessary to heat the thermoplastic film to a temperature close to its melting point in order to generate an initial perforation as will be described in detail later. Further it is necessary to keep higher the temperature of the thermoplastic film in order to enlarge the initial perforation to a target size.
Generally polyester film is used as the thermoplastic film of the heat-sensitive stencil master material. As the polyester, copolymer of ethylene terephthalate and ethylene isophthalate, polyethylene terephthalate, and the like are used. The melting points of these materials are in the range of about 200 to 250xc2x0 C. Accordingly the temperature condition applied to the thermoplastic film in heat-sensitive stencil master making is more severe than that applied to the recording medium such as heat-sensitive paper in thermal recording systems such as thermal transfer recording. When making a heat-sensitive stencil master, the heater elements cannot be microscopically brought into close contact with the thermoplastic film due to their surface roughness and an air layer is formed therebetween, which deteriorates thermal transfer efficiency. Accordingly the heater elements must be heated higher than the thermoplastic film, which results in a heater element peak temperature of 300 to 400xc2x0 C. when the heater elements are continuously operated at cycles of 2 to 4 msec.
Thus, probability of deterioration or breakage of the heater elements has made it difficult to meet the requirements of shortening the stencil master making time and/or improving applicability of heat-sensitive stencil master making to various sensitivities of thermoplastic film in order to obtain a stencil master which is small in fluctuation in shape of the perforations and is high in printing durability and accuracy in printing position.
We observed the manner in which thermoplastic polyester film (employed in a heat-sensitive stencil master material) was perforated upon application of heater drive power to the heater elements of a thermal head in contact with the polyester film and found that the polyester film was perforated in the following two steps. First step was a latent step from initiation of application of heater drive power to generation of an initial perforation and the second step was a growing step during which the initial perforation grew and growth of the perforation stopped.
Polyethylene terephthalate film 1.7 xcexcm thick was employed as the thermoplastic polyester film and the film was perforated by use of a thin film type thermal head. The thermal head was 400 dpi in resolution and 30 xcexcm (in the main scanning direction) xc3x9740 xcexcm (in the sub-scanning direction) in size of each heater element. Heater drive power of 120 mW was applied to each heater element continuously for 400 xcexcsec. The time from the beginning of the dead step to the end of the dead step was about 200 xcexcsec and the time from the beginning of the dead step to the end of the growing step was about 800 xcexcsec. The condition of application of heater drive power was common in current heat-sensitive stencil master making. The shape of the perforation was substantially quite round at any time during the growing step.
FIG. 29 shows the relation of measured power P, heater element temperature a, and size d of the perforation in the main scanning direction with the time t from initiation of application of heater drive power. The size d of the perforation in the main scanning direction is a length of an orthogonal projection of the perforation onto the main scanning axis as shown in FIG. 30.
The temperature of the heater element was measured by applying heater drive power to the heater element under the condition described above without anything in contact with the heater element and by use of an infrared radiation thermometer RM-2A (BARNES ENGINEERING COMPANY) with the field of view set to be a circle 7.5 xcexcm in diameter, a band pass filter whose half-amplitude level was 4.9 to 5.4 xcexcm used, and with the infrared emissivity xcex5 taken as 1. Since wavelengths near 5 xcexcm are in the characteristic absorption band of the glass on the surface of the heater element, the glass may be considered to be a black body and the temperature of the heater element can be calculated on the basis of the radiation intensity.
The result of the experiment described above shows that it takes a time about a half of the duration of heater drive power for an initial perforation to be generated and it takes a time about double of the duration of heater drive power for the perforation to be fixed as measured from the initiation of application of heater drive power. The reason for this fact will be as follows.
When the thermoplastic film is perforated, the thermoplastic film cannot be perfectly brought into close contact with the heater elements and a gap is formed between the thermoplastic film and the heater elements due to the surface roughness of the thermoplastic film itself and/or the surface roughness generated when bonding the thermoplastic film to the support sheet. Since the gap and the thermoplastic film in contact with the gap have a heat capacity, the temperature of the part of the thermoplastic film in contact with the center of the heater element does not change as shown by line a in FIG. 29 representing the temperature of the heater element. That is, though the temperature of the heater element begins to lower as soon as application of heater drive power is stopped, the temperature of the thermoplastic film keeps rising after application of heater drive power is stopped as shown by line b in FIG. 29 and reaches a peak somewhat later than the temperature of the heater element. Thereafter the temperature of the thermoplastic film gradually lowers.
Actually, the part of the thermoplastic film in contact with the center of the surface of the heater element is melted away and there is no part of film in contact with the center of the heater element after the initial perforation is generated. Accordingly, the part of line b representing the temperature of the thermoplastic film in the period after generation of the initial perforation represents imaginary temperatures determined on the assumption that the thermoplastic film is not perforated even if it is subjected to heat. The term xe2x80x9ctemperature of the thermoplastic filmxe2x80x9d as used hereinbelow means the imaginary temperature of the part of the thermoplastic film in contact with the center of the heater element unless otherwise noted. The reason why such imaginary temperatures are used in the following discussion is that the imaginary temperatures are a parameter representing temperature distribution on the thermoplastic film.
When it is assumed that the thermoplastic film is not perforated by heat from the heater element, the thermoplastic film exposed to heat from the heater element has a temperature distribution such that the temperature is the highest at the part in contact with the center of the heater element and lowers as the distance from the center increases as shown in FIG. 31. The temperature distribution on the thermoplastic film shown in FIG. 31 is similar to that on the heater element shown in FIG. 26 though they are different from each other in absolute values of temperature. FIG. 32 shows the cross-sections of temperature distribution on the thermoplastic film and that on the heater element taken along line A-Axe2x80x2 (the main scanning axis, i.e., the center line of the array of the heater elements of the thermal head) at a certain time point. As can be seen from FIG. 32, the temperature of the heater element is higher than that of the thermoplastic film at this time point.
The initial perforation is generated at the part of the thermoplastic film in contact with the center of the surface of the heater element. The temperature of the thermoplastic film at the time the initial perforation is generated will be referred to as xe2x80x9cthe perforation generation temperaturexe2x80x9d, hereinbelow. It has been empirically found that the perforation generation temperature is substantially equal to the melting point of the thermoplastic film. FIG. 33 shows the relation of the temperature of the heater element (line a), the temperature of the thermoplastic film (line b) and the size of the perforation in the main scanning direction (line d) to the time t from initiation of application of heater drive power obtained by simulation. Lines a, b and d in FIG. 33 respectively correspond to lines a, b and d in FIG. 29. The shorter the time in which the temperature of the thermoplastic film reaches the perforation generation temperature Ta, the shorter the time in which the initial perforation is generated.
As the temperature of the thermoplastic film increases after the initial perforation is generated, heat shrinkage of the thermoplastic film occurs near the contour of the initial perforation, and the contour of the initial shrinkage is pulled toward the lower temperature side, whereby the perforation grows. When the heat shrinkage factor of heat-shrinkable film is measured by TMA, heat shrinkage generally begins at a certain temperature Tb (will be referred to as xe2x80x9cthe shrinkage initiation temperaturexe2x80x9d) as shown in FIG. 34. Shrinkage of the film occurs inside the area circumscribed by an isothermal line at the shrinkage initiation temperature Tb in the temperature distribution on the film. The area between the intersections of the main scanning axis A-Axe2x80x2 and the shrinkage initiation temperature line in FIG. 32 will be referred to as xe2x80x9cshrinkage areaxe2x80x9d, hereinbelow. Line c in FIG. 33 shows the shrinkage area at each time.
As shown by line d in FIG. 33, the perforation is generated when the temperature of the thermoplastic film represented by line b reaches the perforation generation temperature Ta and approaches the shrinkage area represented by line c as the temperature of the thermoplastic film approaches the peak. Then the perforation is somewhat enlarged after the temperature of the thermoplastic film reaches the peak.
In FIG. 33, the reason why the size of the perforation represented by line d does not change with the shrinkage area represented by line c before the temperature of the thermoplastic film reaches the peak is that the size of the initial perforation is almost 0 whereas the shrinkage area at the time the initial perforation is generated is a half or more of the final size of the perforation, that the growing speed of the perforation, i.e., the speed at which the contour of the perforation moves, is limited, and that the size of the perforation gradually approaches the shrinkage area.
In FIG. 33, the reason why the size of the perforation does not change with the shrinkage area after the temperature of the thermoplastic film reaches the peak is that the perforation once enlarged cannot be contracted, and the contour of the perforation acts as a heat source which causes heat shrinkage near the contour of the perforation and enlarges the perforation after the temperature of the thermoplastic film reaches the peak.
The shrinkage initiation temperature Tb can be measured by TMA. The perforation generation temperature Ta and the speed at which the perforation grows depend upon physical properties, structure, temperature condition of the thermal head and the thermoplastic film though the dependency has not been known in detail.
Anyway, in the case of the conditions described above, it takes a time about a half of the duration of heater drive power for an initial perforation to be generated and it takes a time about double of the duration of heater drive power for the perforation to be fixed as measured from the initiation of application of heater drive power.
Further in the case of low-sensitive thermoplastic film, it is necessary to apply energy larger than that applied to high-sensitive thermoplastic film in order to obtain a desired shape of perforations. For this purpose, the heater drive power and/or the duration of heater drive power must be increased. This is because the low-sensitive thermoplastic film is higher than the high-sensitive thermoplastic film in the perforation generation temperature, the shrinkage initiation temperature and the temperature at which the initial perforation grows at a predetermined speed.
When the low-sensitive thermoplastic film is applied with heater drive power under the same condition as for the high-sensitive thermoplastic film in the conventional stencil master making apparatus, changes with time t of the temperature of the heater element, the temperature of the thermoplastic film, the shrinkage area and the size of the perforation are as shown by lines a to d in FIG. 35A. As can be seen from line d, the size xcfx86 of the perforation obtained becomes smaller than the target size xcfx860 in this case.
If the duration of heater drive power is increased in order to enlarge the perforation to the target size xcfx860, the temperature of the heater element exceeds an upper limit temperature Tmax below which deterioration of the heater element can be avoided as shown in FIG. 35B. This upper limit temperature Tmax will be referred to as xe2x80x9cthe maximum set temperature Tmax of the heater elementxe2x80x9d, hereinbelow.
The maximum set temperature Tmax of the heater element is based on probability. Though, in FIG. 35B, the maximum set temperature Tmax of the heater element is specified, the maximum set temperature Tmax of the heater element is not a temperature such that deterioration of the heater element is sharply promoted when the temperature of the heater element exceeds the temperature. The heater element is deteriorated to a higher extent as the temperature which the heater element experiences becomes higher.
The perforation can be enlarged to the target size xcfx860 by increasing the heater drive power in place of increasing the duration of heater drive power. However when the heater drive power is increased, the peak of the temperature of the heater element becomes much higher than the maximum set temperature Tmax of the heater element.
As described above, it is generally necessary to shorten the duration of heater drive power in order to shorten the stencil master making time. However in order to shorten the duration of heater drive power, the heater drive power must be increased. That is, the temperature of the thermoplastic film must be quickly increased to the perforation generation temperature Ta so that the perforation is enlarged to the target size xcfx860 in a short time.
FIG. 36A shows changes with time of the temperature of the heater element (line a), the temperature of the thermoplastic film (line b), the shrinkage area (line c), and the size of the perforation (line d) when the heater element is energized with the heater drive power and the duration thereof set at values typical in the conventional stencil master making method. FIG. 36B shows changes with time of the temperature of the heater element (line a), the temperature of the thermoplastic film (line b), the shrinkage area (line c), and the size of the perforation (line d) when the heater drive power is increased and the duration of heater drive power is shortened so that perforation of the target size xcfx860 can be obtained in a time shorter than in the conventional method. As can be seen from FIG. 36B, the temperature of the heater element exceeds the maximum set temperature Tmax of the heater element when the heater drive power is increased and the duration of heater drive power is shortened.
As described above, there has been known a technique in which chopped pulses are applied to heater elements in thermal transfer printing in order to prevent sticking of ink sheets, keep a temperature suitable for printing and control the printing density, as disclosed, for instance, in Japanese Unexamined Patent Publication No. 62(1987)-51465.
As is well known, thermal transfer film employed in the thermal transfer printing comprises a support sheet (generally of polyethylene terephthalate, about 260xc2x0 C. in melting point), a heat-resistant release material layer formed on one side of the support sheet and an ink layer formed on the other side of the support sheet. A thermal head is brought into contact with the release material layer and the ink layer is brought into contact with a recording paper. The support sheet contributes to keeping the ink layer flat and in a uniform thickness. The heat-resistant release material layer prevents the support sheet from being melted and sticking to the thermal head. The ink layer is melted or sublimed and is transferred to the recording paper when heated by the thermal head. The ink layer is transferred to the recording paper in the temperature range higher than the melting point of the ink layer (65 to 75xc2x0 C.) or in the sublimation temperature of the ink layer (140 to 200xc2x0 C.). The support sheet is heated to a temperature higher than the temperature range. If the support sheet is deformed by melting or heat shrinkage in the temperature range, the ink layer cannot be kept flat and fluctuation in transfer occurs, which deteriorates quality of the transferred image. Accordingly the support sheet should be kept in a temperature range where it cannot be deformed by heat. Though such a temperature range has not been precisely known, the melting point of the support sheet can be a parameter. That is, when the temperature of the support sheet is held below the melting point (about 260xc2x0 C.) or so, deformation of the support sheet can be generally prevented. The thermal transfer film is inherently very smooth, and accordingly is very high in thermal transfer efficiency.
To the contrast, the heat-sensitive stencil master material comprises porous support sheet and thermoplastic film laminated on the porous support sheet. Perforations are formed in the thermoplastic film and the porous support sheet strengthens the thermoplastic film and is permeable to ink supplied to the perforated thermoplastic film. As described above, it is necessary to heat the thermoplastic film to a temperature above the perforation generation temperature (substantially equal to the melting point of the thermoplastic film=200 to 250xc2x0 C.) in order to generate the aforesaid initial perforations. Further in order to enlarge the initial perforations to a target size, it is necessary to keep the temperature of the thermoplastic film in a higher range. The thermoplastic film is 1 to 2 xcexcm in thickness and is smooth in itself. The porous support sheet is 30 to 40 xcexcm in thickness and is large in fluctuation of fibers, which makes uneven the surface of the support sheet. Further, since the support sheet and the thermoplastic film are different from each other in elasticity and shrink, the heat-sensitive stencil master material is greatly inferior to the thermal transfer film in surface smoothness. According to our investigation, typical heat-sensitive stencil master material was about 0.4 to 3 xcexcm in arithmetic mean surface roughness Ra of the surface of the thermoplastic film though depending upon the diameter and/or dispersion of the fibers of the support sheet. To the contrast, typical melting type or sublimation type thermal transfer film was lower than about 0.1 xcexcm in arithmetic mean surface roughness Ra. Thus the thermal transfer efficiency from the thermal head to the heat-sensitive stencil master material in stencil master making is greatly inferior to that from the thermal head to the thermal transfer film in thermal transfer printing. The aforesaid arithmetic mean surface roughness Ra was measured by use of a non-contact three-dimensional geometry analyzer NH-3 (Mitaka Optical Instrument) with the cut-off value xcexc set at 0.8 mm and with the evaluation length ln set at 2.34 mm.
As can be seen from the description above, the heater elements of the thermal head are heated much higher in stencil master making than in thermal transfer printing, and accordingly the teachings of Japanese Unexamined Patent Publication No. 62 (1987)-51465 cannot be applied to heat-sensitive stencil master making as they are.
In view of the foregoing observations and description, the primary object of the present invention is to provide a stencil master making apparatus which can meet various requirements from the viewpoint of quality of printed matter without deterioration of heater elements of a thermal head, that is, a stencil master making apparatus which can make a stencil master which is excellent in uniformity in the perforation size, printing durability and accuracy in the printing position in a short time without fear of deterioration of heater elements of a thermal head.
In accordance with a present invention, there is provided a a heat-sensitive stencil master making apparatus which makes a stencil master by imagewise perforating heat-sensitive stencil master material according to an image on an original comprising a thermal head which has an array of a plurality of heater elements and is brought into thermal contact with the heat-sensitive stencil master material, and an electric voltage applying means which applies an electric voltage to heater elements selected from the array of the heater elements according to the image on the original so that perforations are formed in the parts of the heat-sensitive stencil master material in contact with the selected heater elements, wherein the improvement comprises that the electric voltage applying means applies a continuous electric voltage to each of the selected heater elements to heat the heater element to a predetermined temperature in a predetermined temperature range adequate to thermally perforate the stencil master material and then applies an intermittent electric voltage to the heater element so that the temperature of the heater element is held in said predetermined temperature range for a predetermined time interval adequate to thermally perforate the stencil master material.
The predetermined temperature range adequate to thermally perforate the stencil master material and the predetermined time interval for which the intermittent electric voltage is applied are empirically determined taking into account the sensitivity to perforation of the stencil master material used, the size of the perforation to be formed, the thermal transfer efficiency between the heater element and the stencil master material, and the like.
When the electric voltage applying means applies an intermittent electric voltage to the heater element, duty may be either fixed or changed. Duty is defined as the ratio of the duration of an on-time to the sum of the duration of the on-time and the duration of an off-time adjacent to the on-time in the intermittent pulse. Accordingly when the duration of the on-time and that of the off-time are fixed, the duty of the intermittent electric voltage is fixed, and when the durations of the on-time and the off-time are changed with time, the duty changes each time the electric voltage is turned on or off.
It is preferred that the electric voltage applying means applies said intermittent electric voltage so that the temperature of the center of the surface of the heater element during application of the intermittent voltage minus the periodic variation of the temperature of the center of the surface of the heater element due to application of the intermittent voltage is held in a temperature range not lower than the melting point of the thermoplastic film of the heat-sensitive stencil master material and not higher than a maximum set temperature determined for the heater element (e.g., 400xc2x0 C.).
Further it is preferred that the electric voltage applying means applies said continuous electric voltage so that at least one of the following two heating rate conditions is satisfied, one being a condition that it takes 25 xcexcsec or more as measured from the initiation of application for the temperature of the center of the surface of the heater element at a room temperature (e.g., 10 to 30xc2x0 C.) to reach 200xc2x0 C. and the other being a condition that it takes 50 xcexcsec or more as measured from the initiation of application for the temperature of the center of the surface of the heater element at a room temperature to reach 300xc2x0 C. It is further preferred that the electric voltage applying means applies said continuous electric voltage so that at least one of the following two heating rate conditions is satisfied, one being a condition that the temperature of the center of the surface of the heater element at a room temperature reaches 200xc2x0 C. within 150 xcexcsec as measured from the initiation of application and the other being a condition that the temperature of the center of the surface of the heater element at a room temperature reaches 300xc2x0 C. within 300 xcexcsec as measured from the initiation of application.
Further, when the stencil master is made by moving the thermal head relatively to the stencil master material in a sub-scanning direction substantially perpendicular to a main scanning direction which is equal to the direction of the array of the heater elements, and the densities of the picture elements of the stencil master in the main scanning direction and the sub-scanning direction are both in the range of 200 dpi to 800 dpi, it is preferred that the continuous electric voltage satisfies the following formula (1),
                    0.005        ≤                                            v              2                        ⁢                                                            P                  x                                ⁢                                  P                  y                                                                                        rl              x                        ⁢                          l              y                                      ≤        0.015                            (        1        )            
wherein v represents the electric voltage (V) to be applied, r represents the mean resistance (xcexa9) of the heater elements, Px represents the pitches (xcexcm) of the picture elements in the main scanning direction, Py represents the pitches (xcexcm) of the picture elements in the sub-scanning direction, lx represents the length (xcexcm) of the heater element in the main scanning direction, and ly represents the length (xcexcm) of the heater element in the sub-scanning direction.
Further it is preferred that the stencil master making apparatus of the present invention be provided with a preheating means which carries out preheating on at least said selected heater elements before said electric voltage applying means applies said continuous electric voltage, the preheating consisting of the steps of applying a continuous electric voltage to each of the heater elements to heat the heater element to a predetermined temperature in a predetermined temperature range adequate to preheat the stencil master material and then applying an intermittent electric voltage to the heater element so that the temperature of the heater element is held in said predetermined temperature range for a predetermined time interval adequate to preheat the stencil master material.
In this case, it is preferred that the heater elements of the thermal head are divided into a plurality of blocks so that the heater elements are driven block by block, and the preheating means carries out the preheating on the heater elements in one block while the heater elements in one of the other blocks are perforating the stencil master material.
It is preferred that the preheating means applies said intermittent electric voltage so that the temperature of the center of the surface of the heater element during application of the intermittent voltage minus the periodic variation of the temperature of the center of the surface of the heater element due to application of the intermittent voltage is held in a temperature range between the melting point of the thermoplastic film minus 50xc2x0 C. and that plus 50xc2x0 C.
In the stencil master making apparatus in accordance with the present invention, since a continuous electric voltage is applied to the heater element to heat the heater element to a predetermined temperature adequate to perforate the stencil master material (more strictly, the thermoplastic film of the stencil master material) and then an intermittent electric voltage is applied to the heater element so that the temperature of the heater element is held near the predetermined temperature, the temperature of the heater element cannot be raised to an excessively high temperature even if the continuous electric voltage to be initially applied to the heated element is increased. Accordingly, the temperature of the thermoplastic film can be rapidly raised to the perforation generation temperature by increasing the continuous electric voltage initially applied to the heater element without fear of deterioration of the heater element in said oxidation mode. Further by changing the time interval for which the intermittent electric voltage is applied to the heater element according to the sensitivity of the thermoplastic film used, the heat transfer efficiency between the heater element and the thermoplastic film, and the like, the temperature of the thermoplastic film can be raised to a temperature optimum to perforation without being affected by these factors, whereby the size of the perforations can be stabilized and a stencil master which is excellent in printing durability, accuracy in the printing position and printing quality can be made in a short time without fear of deterioration of heater elements of a thermal head.
When the electric voltage applying means applies said intermittent electric voltage so that the temperature of the center of the surface of the heater element during application of the intermittent voltage minus the periodic variation of the temperature of the center of the surface of the heater element due to application of the intermittent voltage is held in a temperature range not lower than the melting point of the thermoplastic film of the heat-sensitive stencil master material and not higher than a maximum set temperature determined for the heater element (e.g., 400xc2x0 C.), deterioration of the heater elements in said oxidation mode can be more surely avoided.
Though the temperature of the thermoplastic film can be raised to the perforation generation temperature in a shorter time as the continuous electric voltage initially applied to the heater element becomes higher, the heater element undergoes excessively sharp temperature change when the continuous electric voltage initially applied to the heater element is too high, which can cause deterioration of the heater element in said crack mode. Accordingly, in order to prevent the heater element from undergoing such sharp temperature change, it is preferred that the electric voltage applying means applies said continuous electric voltage so that at least one of the following two heating rate conditions is satisfied, one being a condition that it takes 25 xcexcsec or more as measured from the initiation of application for the temperature of the center of the surface of the heater element at a room temperature (e.g., 10 to 30xc2x0 C.) to reach 200xc2x0 C. and the other being a condition that it takes 50 xcexcsec or more as measured from the initiation of application for the temperature of the center of the surface of the heater element at a room temperature to reach 300xc2x0 C. Further, in view of shortening the stencil master making time, it is preferred that the electric voltage applying means applies said continuous electric voltage so that at least one of the following two heating rate conditions is satisfied, one being a condition that the temperature of the center of the surface of the heater element at a room temperature reaches 200xc2x0 C. within 150 xcexcsec as measured from the initiation of application and the other being a condition that the temperature of the center of the surface of the heater element at a room temperature reaches 300xc2x0 C. within 300 xcexcsec as measured from the initiation of application.
When the electric voltage applying means applies a continuous electric voltage which satisfies the aforesaid formula (1), the temperature of the heater elements can be increased at a constant rate without being affected by the resolution as will be described in more detail later.
When the heater element is preheated by the preheating means before the main heating (application of the continuous electric voltage for heating the heater element to said predetermined temperature in the temperature range adequate to thermally perforate the stencil master material and application of the intermittent electric voltage for holding the temperature of the heater element in the temperature range), the temperature of the thermoplastic film has been raised to a certain temperature, and accordingly, the temperature of the thermoplastic film can be more rapidly raised to the perforation generation temperature by the main heating and the perforation grows at a higher speed. When the perforation grows at a higher speed, the time for which the intermittent electric current is to be applied to the heater element after application of the continuous electric voltage can be shortened, whereby the time for which the heater element is exposed to high temperature can be shortened, which is advantageous from the viewpoint of preventing deterioration of the heater element in the oxidation mode. Further, when the heater elements of the thermal head are divided into a plurality of blocks so that the heater elements are driven block by block, and the preheating is effected for the heater elements in one block while the heater elements in one of the other blocks are perforating the stencil master material, the total stencil master making time is shortened.